The elementary charge e is a fundamental physical constant with a measured value of approximately 1.602176487(40)×10−19 C. It is the smallest measurable value of the electric charge in stable matter, despite many recent attempts to measure fractional charges such as ⅓ e and ⅔ e. Almost 100 years ago, Robert Millikan carried out the first measurement of the value of e by observing the motion of charged oil drops in air under the influence of an electric field. The present invention relates to the measurement of the elementary charge on solid particles in a liquid. Measuring the elementary charge in a liquid is more difficult than in air because of the higher viscosity, which reduces the motion of weakly charged particles in an electric field to a value which may be below the sensitivity of most measurement systems.
The rationale for accurate measurement of small charges is the need for characterization of colloids, for determining the charge and size distribution of colloids, for studying ionic reactions at the surface of particles and for the detection of low concentrations of molecules, where in the ideal case the binding of a single molecule can be registered.
A number of exemplary methods for determining particle characteristics such as for example charge or size is already available on the market. For example, U.S. Pat. No. 6,281,972 describes a method for measuring the size distribution of particles in a gas. The method is based on selecting or classifying the mobility of a plurality of particles and measuring the size of a number of particles. For the different size measurements, the mobility is kept fixed. There still is a need for accurate and/or sensitive measurement techniques for detecting properties of particles, such as for example size or charge.